Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a first image of a sequence of a display screen or portion thereof with a transitional graphical user interface showing the new design.

FIG. 2 is a second image in the sequence thereof; and,

FIG. 3 is a third image in the sequence thereof.

The subject matter in this patent includes a process or period in which an image changes into another image. This process or period forms no part of the claimed design. The dashed broken line showing of a display screen or a portion thereof is for the purpose of illustrating portions of the article and forms no part of the claimed design. All other broken line showing text and other elements of the graphical user interface is for the purpose of illustrating portions of the article and forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 